The invention relates to an X-ray source having positively cooled rotating anode, as claimed in the precharacterizing clause of patent claim 1.
In X-ray tubes based on rotating piston tubes, the entire tube is mechanically kept rotating quickly, and the electron beam is mechanically fixed on the focus. The space between the tube and source housing in such known X-ray tubes (DE 197 41 750 A1) is filled with a suitable liquid coolant, generally oil. The oil filling is used firstly to dissipate the amount of heat produced at the anode, and secondly to provide sufficient isolation for the high voltages, positive on the anode and negative on the cathode, from the source housing, which is at ground potential. Such a closed system with a global oil filling results in a number of problems.
Firstly, the rotation power that needs to be used for the rotation speeds that are required nowadays ( greater than 100 revolutions per second) are considerable owing to the very high friction losses; if at all, such power can be reduced only by comparatively high design complexity. Secondly, the oil filling must be introduced into the tube housing very carefully in order to avoid the formation of bubbles. This is because, during tube operation, the formation of vapor bubbles due to cavitation in the isolation area can lead to considerable problems in terms of the ability to withstand high voltages. The dissipation of heat losses from the rotating anode is also problematic.
The known X-ray source which has been mentioned above attempts to solve this problem by providing an external heat exchanger for cooling down the oil and by arranging the inlet and outlet for the oil at points on the source housing at which a reduced pressure or increased pressure is produced by the rotation of the rotating piston.
A rotating piston tube whose anode is provided with ribs on its outside is known from DE 8 713 042 U1. A coolant, in particular a liquid coolant, is applied to the outside of the anode.
As an alternative to cooling the anode using oil, it is also known for the anode to be cooled using a cooling gas, in which case the tube can be provided on its outside with circular ribs to improve the heat dissipation, and these can also be used at the same time for the tube drive (EP 0 187 020 B1).
In conjunction with an X-ray tube having a fixed anode, U.S. Pat. No. 4,418,421 refers to a prior art which states that gas cooling using sulfur hexafluoride (SF6) can be provided, instead of oil cooling of the anode, in order to save weight. However, it is regarded as a disadvantage of such designs that the comparatively large amount of heat produced in particular from high-power tubes in the range from 70 to 100 kV can lead to a reduction in the insulation capability of the gas.
The cited US Patent Specification also refers to a further prior art, which states that the two media, oil and gas, can be physically separated and the X-ray tube can be accommodated in a first housing filled with oil, with the high-voltage parts being arranged in a second housing in which the gas filling is introduced. Although the two separate housings are electrically and mechanically connected to one another, the two media are arranged such that they are isolated from one another. However, any design for this purpose is comparatively complex.
The invention specified in patent claim 1 is based on the object of specifying an X-ray source of the type mentioned initially which allows the disadvantages of the known X-ray sources to be avoided. In particular, the X-ray source is intended to ensure reliable isolation of the high-voltage parts irrespective of the rotation speed and to have low friction losses, so that the X-ray tube can be operated at a higher rotation speed and with less friction losses than in the past.
According to the invention, the functions of electrical isolation and cooling of the anode are separated, but without having to physically separate from one another the two media provided for this purpose in the tube housing. The insulating gas is incorporated in the source housing such that it is not physically separated from the cooling routing.
In order to achieve approximately the same isolation capability as with oil, the high-voltage parts advantageously have sulfur hexafluoride (SF6) applied to them at a gas pressure of approximately 3 bar. In these conditions, the gas is an excellent insulator, and is chemically completely inert up to several hundred degrees Celsius.
The coolant is cooled in an open cooling system so that there is no need for any expansion vessel and the exchange between the tube and coolant is simplified. Oil is preferably used for cooling the thermally highly loaded anode plate and is supplied, by a feed pump, from a reservoir concentrically with respect to the bearing shaft of the anode, is then first of all positively guided via narrow gaps along the tube outside of the anode plate and along the beam outlet window, and is then passed out radially into the source housing via a baffle plate, or possibly a number of baffle plates, which is or are arranged on the tube. The reservoir is advantageously arranged inside the source housing and is in the form of an open sump. Together with the feed pump, the sump can also be arranged outside the source housing, and may be in the form of a heat exchanger.
One major advantage of the arrangement according to the invention is that the X-ray source has only a fraction of the friction losses known from previous sources, so that the tube can be operated with reduced friction losses at comparatively high rotation speeds.
The guide body, which is arranged fixed in the source house, can advantageously be at least partially formed by walls of the source housing itself, provided the design conditions allow. In order to optimize the way in which the coolant, which is sprayed out on the baffle plate, is carried away, the guide body can be provided with a shroud-like guidance element, which advantageously has a semicircular cross section. If necessary, there may be a number of baffle plates on the rotating piston tube, engaging like laminates in the stationary guide body. The baffle plates or roe laminates are advantageously at least partially composed of elastic material, and the elastic parts rest axially and/or radially on the corresponding surfaces of the guide body. This ensures that as little coolant as possible can enter the remaining part of the source housing. The baffle plates, which are provided like laminates, need provide only sealing against oil spray.
The cooling system is advantageously in the form of an open cooling system, with oil preferably being used as the coolant which, using a pump, is first of all passed with positive guidance from an open oil sump to the parts to be cooled, and then flows back into the oil sump again without positive guidance.